PROJECT SUMMARY/ABSTRACT Systemic administration of high-dose IL-2 has been used since the 1980?s as an FDA-approved immunotherapy for metastatic cancer. Despite the fact that up to 9% of patients treated with high dose IL-2 achieve a durable, long term response, this therapy is rarely used today due to significant life-threatening complications. Such complications occur due to IL-2 activation of vascular endothelium, resulting in systemic capillary leak, as well as other adverse effects associated with ?off target? signaling of IL-2. In addition, IL-2 preferentially activates CD4+Foxp3+ regulatory T cells that mitigate the tumor-specific response. We recently developed an IL-2 fusion protein which targets and solely activates cytotoxic T lymphocytes in the absence of ?off target? side effects. In addition to a dramatically increased safety profile, our IL-2 fusion protein inhibits the growth of highly aggressive tumors normally resistant to other forms of immunotherapy. The purpose of the current proposal is to perform pre-IND risk mitigation studies to advance this fusion protein to the clinics. Successful completion of the proposed studies will support an Investigational New Drug Application (IND) to the FDA and lay the basis for a Phase I safety feasibility trial in man.